T121_Baghouse_Fines_&_Dust_Control.pdf

TECHNICAL PAPER T-121

TECHNICAL PAPER

T-121

BAGHOUSE FINES AND DUST CONTROL OPTIONS

By J. Don Brock, PhD., P.E. Revised by Jonathan Brown and Greg Fricks

TECHNICAL PAPER T-121 BAGHOUSE FINES AND DUST CONTROL OPTIONS By J. Don Brock, PhD., P.E. Revised by Jonathan Brown and Greg Fricks

TABLE OF CONTENTS INTRODUCTION ......................................... 2

Surge Pots.............................................. 8

MIX DESIGN ................................................ 2

Weigh Pots ............................................. 8

Effects of Dust in the Mix ........................ 2 Mix Design ................................................ 3 DUST CARRYOUT ...................................... 3 COLLECTION EQUIPMENT ...................... 5 Primary Collectors ................................... 6

Inertial Separator .................................... 6 Cyclones................................................. 6 Multicone ................................................ 6 Wet Scrubber ......................................... 6

Batch Tower Bin 1 .................................. 8 Batch Tower Mixer .................................. 9 Continuous Mix Facilities........................ 9 DUST CONTROL SYSTEMS ................... 10 AC and Duct Control General Flow  ...... 10

Option 1 Total Control ........................... 12 Option 2 Split Return with Weigh Pot ... 12 Option 3 Split Return ............................ 13

Baghouses ................................................ 6

Option 4 Split Return with Surge Pot .... 13

Reverse Air ............................................. 7

Option 5 Return All with Surge Pot ....... 14

Reverse Pulse ........................................ 7

Option 6 Return All ............................... 14

Pulse Jet................................................. 7

Available Dust Return Options: Batch  . 15

Dryer Dust Collecting Systems: Components ............................................. 7

Baghouse Discharge Points ................... 7 Surge and Weigh Pots ........................... 8

1

Dust Return Locations............................ 8

Return All .............................................. 15 Split Return with Auger ......................... 15 Split Return with Weigh Pot .................. 16 CONCLUSION ............................................ 16

INTRODUCTION  Aggregates used to produce hot and warm mix asphalt contain varying gradations of material, ranging in size from one and one-half inch down to less than one micron, a ratio of 38,000 to one. Some of the ner particles become airborne during the drying process and are collected in various types of collection systems. As air pollution requirements became more stringent in the mid-1970s, dry fabric lter dust collector baghouses became the prevalent collector used on asphalt facilities. The ne dust particles collected in these devices are commonly referred to as baghouse nes. Baghouse nes are not foreign ingredients used in the mix, but merely the ner fraction of the aggregate being processed. The size of the particles collected in the baghouse is determined by three factors: 1. The actual gradation of the original material being processed 2. The sequence of collectors used to collect the dust before the baghouse 3. The amount of degradation of material through the process

MIX DESIGN  An integral part of mix design is determining the proper dust-to-asphalt ratio. Similarly, an integral part of mix production is the proper handling and control of dust throughout the process. Because 25-50% of the dust that is fed into the dryer can become entrained into the gas stream and captured by dust collection equipment, the method by which dust is accounted and controlled is very important, since the nal mix gradation can quickly move out of specication. Knowing how to reintroduce the proper amount of this dust back into the production cycle will ensure that the mix produced continues to stay within specications, with the least amount of equipment and labor costs. EFFECTS OF DUST IN THE MIX

Before air pollution laws changed, a minimum amount of the material smaller than 200 mesh was returned to the mix. In the early 1980s, NAPA’s Quality Improvement Committee conducted an exhaustive study on using baghouse nes in asphalt mix to learn whether they were detrimental or benecial. The study found that adding dust particles smaller than the lm thickness of the asphalt to the mix was benecial. The dust particles extended the asphalt, allowing use of less liquid asphalt.

NAPA found that such dust, generally less than 15 microns in size, could virtually replace asphalt, but if used in excess, was detrimental and would stiffen the asphalt mix considerably. Accordingly, NAPA recommended a specic ratio of dust to asphalt. A ratio by mass of 1 part dust (up to 1.2 parts dust) smaller than 15 microns to 1 part liquid asphalt was recommended. Specifying agencies have adopted this dust to asphalt ratio and have now included it in many specications. They broadly refer to material smaller than 200 mesh without designating its specic makeup. This is in error. More than 1 part of dust can be satisfactorily used, depending on Excessive Dust •

Increases surface area, decreases lm thickness and durability

•

Increases AC requirement

•

Can become part of the asphalt causing hardening

Insufcient Dust •

Decreases surface area, increases lm thickness and results in tender mix

•

Results in higher air voids, increasing oxidation and moisture problems

2

the makeup of dust smaller than 200 mesh (75 microns). But if the dust contains substantially high percentages of particles smaller than 15 microns, the 1 to 1 ratio is correct. For a fuller explanation of this study please refer to NAPA’s QIP 102.

MIX DESIGN

Each mix design starts with a specication with minimum and maximum limits for each gradation size (Figure 1). Gradation limits help ensure that the mix gradation is maintained so that desired performance properties are achieved.

FIGURE 1

Using currently available aggregates, the contractor determines a Job Mix Formula (JMF) with upper and lower tolerances for use on the project (Figure 2). The JMF is the blend of aggregates best suited to achieve an actual mix gradation that falls within the gradation limits of the mix design. Tolerances provide a mechanism by which the JMF may be monitored and controlled to consistently provide the desired mix design within minimum and maximum limits for each gradation size.  Aggregate supplies that contain too much dust can cause the nal mix to go out of specication by exceeding the maximum allowable dust levels. For aggregate supplies that contain too little dust, or in instances where too much dust is removed, the nal mix will be out of specication for too little dust.

FIGURE 2

DUST CARRYOUT To ensure that the liquid asphalt will adhere to the dust particles, the aggregate must be dried and heated to the desired mix temperature. To accomplish the drying process, the aggregate is fed into a rotating drum where it is picked up and cascaded through a stream of hot gases. The size of the dust particles that become airborne vary depending on the velocity of gases in the drum (drum gas velocity) as well as the design of the AC injection point. Typical aggregate dryers (Figure 3), such as those found on batch plants, have a drum gas velocity limit of approximately 800 to 1,000 feet per minute. 3

FIGURE 3: Typical batch plant aggregate drum dryer

For early parallel ow drum mixers (Figure 4), liquid asphalt cement is injected downstream in the drum so that the mixing occurs away from the drying section. Less dust is carried out by the gas stream because the asphalt causes the dust to adhere to the aggregate. With this design, drum gas velocities were increased to approximately 1,000 feet per minute.

FIGURE 4: Parallel flow drum dryer/mixer

 A drum with a coater or mixer at its discharge end (Figure 5) has design gas stream velocities of approximately 1,000 feet per minute and higher. Dust carryout occurs because the asphalt cement is not injected directly into the drum. However, the large dropout area above the coater causes larger airborne dust particles to fall out of the airstream and into the coater.

FIGURE 5: Parallel flow drum dryer with coater

 A counter ow drum mixer with an embedded burner (Figure 6) operates at similar drum gas velocities as the counter ow dryer.

FIGURE 6: Counter flow drum dryer/mixer

The Double Barrel ® mixer (Figure 7) incorporates a counter ow dryer comparable to that shown in Figure 6 and has drum gas velocities of approximately 1,000 feet per minute.  As aggregate travels through a dryer, dust particles of various sizes are picked up by the gas stream. The gas velocity at which these particles become airborne is called the terminal velocity. The larger the particle, the higher the gas velocity required to make it become airborne. FIGURE 7: Double Barrel ® drum dryer/mixer

4

E XHAUST

BURNER

DAMPE R DRUM MI XE R

BAGHOUSE

TO DAMPE R

FROM DRUM DRAFT CONTROL

FIGURE 8: Draft control system

Draft controls have been installed on plants (Figure 8) to make them more energy efcient. These systems sense the suction at the front of the drum, adjacent to the burner. The controls open or close a damper or change the speed of a Variable Frequency Drive (VFD) on the exhaust fan to maintain a slight negative constant pressure or suction at the burner. The drum gas velocities of 800 and 1,000 feet per minute mentioned above are typically the maximum velocities for which plants are designed and correspond to plants’ maximum production capacity.

COLLECTION EQUIPMENT The coarse fines, typically above 200 mesh (75 μm), are controlled independently of smaller nes (Figure 9). The ability to independently control particles smaller than 200 mesh gives signicant exibility to the operator in order to meet the dust gradation requirements. Coarse nes from the primary collector, such as a cyclone or inertial separator, fall into a collecting hopper. After passing through a discharge point seal, the coarse nes are picked up by an inclined screw conveyor and returned to the drum or mixer.

Fine dust is collected on the outside of aramid bags, dropped into the hopper section and then removed from the baghouse by a screw auger. From there, these nes can be wasted, stored, or returned to the mixer. If a portion (0 to 100%) of these nes is to be wasted, they are conveyed by another screw conveyor to a truck or collection point for waste. Portions not wasted are returned to the mixer together with the coarse nes. A silo can be used to store the nes, instead of being wasted.

dirty air

ne dust

clean air course dust

FIGURE 9: General air and dust flows in an ASTEC Baghouse

5

The size of dust particles leaving the drum ranges from 30 mesh down to less than one micron. For comparison, human hair is approximately 100 microns in diameter; 200 mesh material is 75 microns; and cigarette smoke is 0.3 micron.

PRIMARY COLLECTORS Inertial Separator: An inertial separator (Figure 10) mounted at the baghouse inlet removes the majority of dust from the airstream prior to the lter bags.

Cyclones perform equally well in both vertical and horizontal congurations. However, horizontal cyclones (Figure 11) are usually preferable, as they are more compact and easily accessible for inspection and maintenance. Multicone: Figure 12 shows a multicone dust collector using a series of small cyclones. Its efciency is higher than the single cyclone described above. This type of collector collects much smaller particles than a single cyclone.

Inertial separators depend on rapid changes in both gas stream velocity and ow direction to remove coarse particles from the air stream.

FIGURE 12: Typical Multi-Cone Dust Collector

FIGURE 10: Typical Inertial Separator

Cyclones:  A cyclone (Figure 11) is congured to produce a cyclonic air ow pattern. The centrifugal force causes the dust to move to the outside of the cyclone. The dust then slides down the wall of the cyclone to the hopper.These devices are very efcient in removing larger particles, but their efciency drops signicantly for particles sizes below 30 microns or 400 mesh.

Wet Scrubber: Although baghouses have replaced them, higher pressure venturi scrubbers, as shown in gure 13, can be used to collect even ner particles than those collected by the low pressure cyclone washer. But they require considerable horsepower to overcome their high pressure drops.

FIGURE 13: Typical Wet Scrubber FIGURE 11: Typical Cyclone Separator (horizontal)

6

BAGHOUSES

When using a baghouse collector, dust is collected on the outer surfaces of lter bags. The bags are automatically cleaned at regular intervals by forced air (Figure 14) causing them to expand and break free the dust caked on their outer surfaces.

FIGURE 15: Indexed Reverse Pulse Cleaning

FIGURE 14: Bag Cleaning

The bags are normally constructed of aramid bers which are felted onto a scrim 1. The felt is then made into a sock-type bag. Each bag ts over a wire cage for support. The bags and cages t into a tube sheet, which is the top wall of the baghouse. Reverse Air and Reverse Pulse: Reverse air and reverse pulse baghouses are separated into different compartments. Separating the lter rows into different compartments allows for continuous operation even while cleaning Reverse Air: In reverse air baghouses, exhaust airow is stopped in the compartment to be cleaned by a damper and the reverse air fan forces clean air back through the lters. Reverse Pulse: The patented Dillman Reverse Pulse process utilizes a rotating nozzle assembly (Figure 15) with timed indexing and fast acting air doors. After aligning the nozzle to isolate a single row of bags, the air door quickly opens, providing a pulse of air, dislodging the dust from the bags. Then the air door is closed, which keeps the row off line, allowing the dislodged dust to fall away from the bags. A dedicated air compressor or separate reverse air fan is not required, because the exhaust fan provides the air for cleaning. Pulse Jet: The pulse jet cleaning system (Figure 16) directs a burst of compressed air through a blowpipe into the top of a row of bags. As the

FIGURE 16: Pulse Jet Baghouse

compressed air passes through the venturi, at the top of the bag cage, process air is induced into the bag. The combination of process air and induced air "pop" the dust off of the bags. Each pulse is controlled by the timed opening and closing of solenoid valves. One blowpipe is positioned above each row of bags and typically two rows of bags are cleaned simultaneously. Bags are not taken ofine for the cleaning process. DRYER DUST COLLECTING SYSTEMS - COMPONENTS Baghouse Discharge Points: Various types of devices (Figures 17 to 20) are used at discharge points to facilitate the movement of dust from the baghouse back to the system, to either be stored or wasted. The rotary airlock or motorized tipping valves are generally used when returning the dust into a continuously pneumatic return system using higher suction. Surge and Weigh Pots: Surge and weigh pots are optional equipment that are added based on operator preference or state specications. They help maintain a greater level of control over the nal mix.

1 See Astec Technical Paper T-139 "Baghouse Applications" for further information. 7

FIGURE 17: Rotary  Air Lock - Con tinusouly discharges the dust from a collecting device. It has rotating vanes in a close fitting housing (like a rotating hotel door).

FIGURE 17: Rotary Air Lock

FIGURE 18: Gravity Chute - The dust prevents reverse flow on this device. However, this device is only effective with components where the suction is low (less than 2 inches of water).

FIGURE 18: Gravity Chute

Surge Pots: Surge pots funnel the ne dust into a rotary airlock which meters the dust through the system. As the amount of dust leaving the baghouse varies (either through cleaning cycles or variations in aggregate sources), the surge pot serves to provide for an even, continuous ow of dust. The speed of the airlock can be adjusted either manually or automatically. Weigh Pots: As dust enters the weigh pot, the control system monitors the weight of the dust via load cells. This allows the operator to know exactly how much ne material is owing through the system. When the set point is achieved, a buttery valve opens, moving the nes through the system. DUST RETURN LOCATIONS

There are many different locations for returning dust back to the system, depending on the plant type, level of control needed, operator preferences or local specications. Batch Tower Bin 1: Dust can be conveyed to the bucket elevator by a dust screw (Figure 21). However, the ne materials will not spread

FIGURE 19: Gravity Tipping Valve - This works like a double door entrance in a hotel or office. The weight of the dust automatically opens the valves letting dust ow from one to

the next, alternately opening and closing. FIGURE 19: Gravity Tipping Valve

FIGURE 21: Dust Slide

FIGURE 20: Pnematic Tipping Valve - This operates like the gravity valve, except either air cylinders or motor-driven linkages automatically open and close the gates in sequence. FIGURE 20: Pnematic Tipping Valve

FIGURE 22: Dust Pan

8

across the screens but will drop directly through all of the screens and collect along one wall of the rst bin, remaining segregated from the rest of the material. A solution is a carrier pan that centers the dust in bin 1 (Figure 22). Batch Tower Mixer: When using a storage bin or silo on a batch tower, the dust can be conveyed directly to the weigh batcher with a screw conveyor (Figure 23). A screw conveyor is more accurate than weighing a small quantity of dust in a large weigh hopper using a scale designed for large quantities. If the dust silo

On counterow dryers (Figure 26), the dust enters into the mixing section downstream of the burner. Since this area has low gas velocities, the dust is not picked up by the airstream. The mixing chamber on the Double Barrel ® (Figure 27) also has low gas velocities, which prevents entrainment of the dust.

FIGURE 25: Parallel Flow Drum Mixer Dust Return System

FIGURE 23: Dust Blower with Dust Silo

FIGURE 26: Counterfl ow Drum Mixer Dust Return System

FIGURE 24: Dust Return with Weigh Batcher

has a weigh pot (Figure 24), the dust can be accurately weighed before it is transferred into the weigh batcher. Continuous Mix Facilities: When returning dust in continuous mix plants, care must be taken to ensure the dust isn't entrained in the airstream.

For parallel ow drums (Figure 25), the dust can enter through a receiver that uses an impinging cone. The liquid asphalt also enters through the receiver, helping to capture the dust.

9

FIGURE 27: Dust Return to Double Barrel® Mixer

DUST CONTROL SYSTEMS The specications set the AC and dust requirements for the mix design. A typical requirement for AC content is 5% of nal mix, with tolerances +/- 0.5%. The ne dust content (sieve size 200) requirement is typically 4-6% passing by weight, with tolerances approximately +/- 2%. Though it is possible to remove signicant quan tities of dust via washing (wet process) and high frequency screening (dry process), each adds to the per ton mix cost. The wet process in particular, adds water to the feed aggregate that will result in higher drying cost and may have other impact on the production process. For this reason it is desirable to use available aggregate and manage nes content in the feed material by balancing the job mix formula to minimize or eliminate the need to waste dust. Furthermore, minimizing virgin material dust content allows for a greater dust contribution from RAP if used. Since ne RAP has higher liquid content due to its large surface area, use of ne RAP is preferable to reduce virgin binder content in as much as the resultant binder grade will allow.  As the aggregate travels up the conveyor, a weigh bridge measures all of the aggregate owing into the drum (measured in tons per hour) and labeled ATPH (Figure 28). This value includes coarse and ne dust. The controller accounts for slight variations in the aggregate ow rate and adjusts the AC ow rate accordingly, to maintain the appropriate AC content. If any dust is removed (DO TPH), then the total amount of aggregate entering the drum is reduced

Mix AC = 5%

Weigh Bridge A TPH

A TPH Drum

A TPH

5%

A TPH x 5%

A TPH x 5%

Controller @ 5%

FIGURE 28: Return All

AC Pump

(ATPH - DOTPH = ADTPH) (Figure 29). However, if the dust is not measured, the control system will use the last known ow rate as measured at the weigh bridge for AC ow rate control. This will result in AC content being different from actual  AC content.

Mix AC = 5%

Weigh Bridge AD TPH

A TPH

Drum

A TPH

5%

Controller @ 5%

DO TPH

A TPH x 5%

A TPH x 5%

AC Pump

FIGURE 29: Remove Without Weighing

If no method of weighing dust is installed, then  AC and dust ow rates (removed or added) are set by manual trial and error adjustments. AC ow rates entered into the control system could be articially higher or lower than the Job Mix Formula level, based on the predicted results of the tests. This leads to a discrepancy between what the control system and test results show as the resulting AC content. For complete and automated control (Figure 30), the dust must be weighed both as it is removed and added back into the process ow, DO TPH and DITPH , respectively. This ensured that the aggregate going to the drum is known. The weigh bridge measures all of the aggregate owing into the process and Weigh Pot 1 measures all of the dust removed, relaying this information to the controller. Using the Job Mix Formula, the controller calculates the amount of dust needed to be returned to the system and the amount of AC needed. (ATPH - DOTPH + DITPH = ADTPH) With slight variations in the amount of total aggregate (A TPH) and dust removed (DO TPH), the controller makes the needed adjustments 10

Mix AC = 5%

Weigh Bridge AD TPH

A TPH

Drum DOTPH

A TPH

DOTPH

weigh pot 1

DI TPH weigh pot 2

ADTPH x 5%

silo

5% DI TPH A TPH x 5% Controller @ 5%

AC Pump

FIGURE 30: Automatic Control

in the amount of dust returned and AC injected, reducing the need for extra tests for trial and error adjustments. In recent years, there is a growing trend for regulating authorities to impose specifications and constraints as well as documentation requirements further upstream in the HMA / WMA production process. The intent of these actions is to help ensure nal mix quality and to help prevent the signicant impact of producing and placing mix that does not meet specications. The rationale is that monitoring and controlling the production process further upstream minimizes the potential downstream effect. Automatic control is the only system that accounts for all dust in the feed materials and adjusts AC ow rate accordingly.

11

Since aggregate sources, aggregate quality, and regulations may change over time, it is important to maintain process exibility. Though automatic control of dust may not currently be necessary in any particular locale, it may become necessary in the future. Conguring dust systems to at least be ready for automatic control as conditions warrant is a wise equipment choice. Configurations may be designed to minimize initial capital outlay based upon current need while making provision for additional equipment should future need arise. For all systems congured to waste dust, it is important to remember that an aggregate dryer is not a classier. Though the process of drying aggregate tends to liberate dust from the aggregate, the amount that can be removed cannot be predicted with any certainty. Aggregate consistency and composition, moisture content, clay content, particle shape, and possibly other aggregate characteristics signicantly affect dust carryout. If dust carryout does not sufciently reduce dust content, then cold feed material (RAP and/or virgin aggregate) dust content must be reduced prior to the drying process by balancing materials, using alternative materials, or removing dust using wet or dry technologies.

AVAILABLE DUST CONTROL OPTIONS - CONTINUOUS

Baghouse

Baghouse

Drum Dryer 

Silo

Drum Dryer 

Silo

Weigh Pot Constant Speed Vane Feeders Coarse/Fine Dust Combined Dust Blower 

Coarse Dust

Constant Speed Vane Feeders

Surge Pot Automatically Adjusted Variable Speed Vane Feeder 

Manually Adjusted Variable Speed Vane Feeder 

Fine Dust

Coarse/Fine Dust Combined

Dust Blower 

Coarse Dust

Automatically Adjusted Variable Speed Vane Feeder 

Fine Dust

Commonly referred to as a “Illinois run-around system”

Option 1 - Total Control

Option 2 - Split Return w ith Weigh Pot

Only system that accounts for all dust in system to automatically calculate required AC content

Operation •

Constant volume vane feeder returns all coarse dust to drum

Operation •

Constant volume vane feeder returns all coarse dust to drum

•

Constant volume vane feeder moves all ne dust out of baghouse

•

Constant volume vane feeder returns all ne dust to weigh pot

•

Blower returns all dust to silo for storage until being returned or wasted

•

Fine dust removed is weighed and sent to surge pot

•

•

Blower returns all dust to silo for storage until being returned or wasted

Fine dust is metered using a mass ow weigh pot utilizing an automatically adjusted VFD air lock

•

•

Fine dust is metered using a mass ow weigh pot utilizing an automatically adjusted VFD air lock

Dust is wasted as needed based on amount in silo

 Advantages •

 Allows for wasting of dust if aggregate blend cannot meet spec

•

More dust can be added than is removed, if needed, because of silo

 Advantages •

Weighing of ne dust leaving baghouse provides amount of fine dust needed for return

•

 AC ow is automatically adjusted based on measured change in ne dust component

•

More dust can be added than is removed, if needed, because of silo

•

Ensures steady dust ow from baghouse at start up and mix design transitions

Disadvantages •

Highest cost

•

Highest maintenance

Disadvantages •

 Amount of dust moved to silo is unknown

•

 Amount of dust to return requires trial and error 

•

 AC ow rate requires trial and error 

•

Since the ne dust is not metered as it leaves the baghouse, required change of  AC ow is unknown

12

Baghouse

Baghouse Drum Dryer 

Drum Dryer 

Surge Pot

Coarse/Fine Dust Combined Coarse/Fine Dust Combined

Constant Speed Vane Feeder 

Manually Adjusted Variable Speed Vane Feeder 

Coarse Dust

Constant Speed Vane Feeders

Manually Adjusted Variable Speed Vane Feeder 

Fine Dust

Fine Dust

Option 3 - Split Return

Option 4 - Split Return with Sur ge Pot

Operation

Operation

•

Constant volume vane feeder returns all coarse dust to drum

•

Constant volume vane feeder returns all coarse dust to drum

•

Manually adjusted VFD vane feeder adjusts ow of ne dust — full range of ow between returning all to drum and wasting all — but does not meter dust ow

•

Constant volume vane feed moves ne dust to surge bin

•

Buttery valve above surge pot closes to waste excess dust when high bindicator senses full surge pot

•

Manually adjusted VFD vane feeder below surge pot adjusts ow of ne dust but does not meter dust ow

 Advantages •

 Allows for wasting of dust if aggregate blend cannot meet spec

 Advantages Disadvantages • •

13

 Amount of dust to waste requires trial and error 

•

 Allows for wasting of dust if aggregate blend cannot meet spec

•

Ensures steady dust ow from baghouse at start up and mix design transitions

Cannot add dust without additional mineral feed system

•

 AC ow rate requires trial and error 

•

Since the ne dust is not metered as it leaves the baghouse, required change of  AC ow is unknown

Disadvantages • •

 Amount of dust to waste is trial and error  Cannot add dust without additional mineral feed system

•

 AC ow rate is trial and error 

•

Since the ne dust is not metered as it leaves the baghouse, required change of  AC ow is unknown

Coarse/Fine Dust Combined

Coarse/Fine Dust Combined Baghouse

Baghouse

Drum Dryer 

Drum Dryer 

Surge Pot Manually Adjusted Variable Speed Vane Feeder 

Constant Speed Vane Feeder  Constant Speed Vane Feeders

Option 5 - Return All with Surge Pot

Option 6 - Return All

Operation

Operation

•

Single constant volume vane feeder returns all dust to surge pot

•

Manually adjusted VFD vane feeder below surge pot returns dust to drum

 Advantages •

Ensures steady dust ow from baghouse at start up and mix design transitions

•

Relatively low cost

Disadvantages •

Coarse and ne dust handled together 

•

Does not control dust in mix beyond cold feed bin controls

•

•

Single constant volume rotary vane feeder returns all dust collected to drum

 Advantages •

Lowest cost

Disadvantages •

Coarse and ne dust handled together 

•

Does not control dust in mix beyond cold feed bin controls

•

 All dust collected will be returned to mix

•

Cannot waste dust

•

Cannot add dust without additional mineral feed system

 All dust collected will be returned to mix

•

Cannot waste dust

•

Cannot add dust without additional mineral feed system

14

AVAILABLE DUST RETURN OPTIONS – BATCH

Option 1 - Return All

Option 2 - Split Return with Auger 

Operation

Operation

•

Single constant volume rotary vane feeder returns all dust collected to bucket elevator 

 Advantages •

Constant volume vane feeder returns all coarse dust to elevator 

•

Constant volume vane feeder moves all ne dust out of baghouse

•

Blower returns all dust to silo for storage until being returned or wasted

•

Fine dust is metered to mixer using auger 

•

Dust is wasted as needed based on amount in silo

Lowest cost

Disadvantages •

Coarse and ne dust handled together 

•

Does not control dust in mix beyond cold feed bin controls

•

•

 All dust collected will be returned to mix

•

Cannot waste dust

•

Cannot add dust without additional mineral feed system

 Advantages •

 Allows for wasting of dust if aggregate blend cannot meet spec

•

More dust can be added than is removed, if needed, because of silo

Disadvantages •

15

 Amount of dust moved to silo is unknown

•

Higher cost than “return all”

•

Higher maintenance than “return all”

•

Blower returns all dust to silo for storage until being returned or wasted

•

Fine dust is metered to mixer using a mass ow weigh pot

•

Dust is wasted as needed based on amount in silo

 Advantages •

 Allows for wasting of dust if aggregate blend cannot meet spec

•

More dust can be added than is removed, if needed, because of silo

Option 3 - Split Retur n with Weigh Pot

Operation • •

Constant volume vane feeder returns all coarse dust to drum Constant volume vane feeder moves all ne dust out of baghouse

Disadvantages •

 Amount of dust moved to silo is unknown

•

Higher cost than “return all”

•

Higher maintenance than “return all”

CONCLUSION Baghouse nes are an integral part of any mix design and should be properly handled and controlled. Understanding how nes move through the system, along with proper measuring and reintroduction methods will allow the operator to produce a consistent, high quality mix.

16

 ASTEC encourages its engineers and executives to author articles that will be of value to members of the hot mix asphalt (HMA) industry. The company also sponsors independent research when appropriate and has coordinat ed j oint authorshi p be tween industry competitors. Information is disbursed to any interested party in the form of technical papers. The purpose of the technical papers is to make information available within the HMA industry in order to contribute to the continued improvement process that will benefit the industry.

17